This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-384823, filed Dec. 18, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a printed wiring board having a plurality of pads arranged like a grid, surface mount circuit component, like a ball grid array (BGA) type semiconductor package, mounted on the printed wiring board, and a circuit module with the circuit component mounted on the printed wiring board.
2. Description of the Related Art
Increased integration and performance of a semiconductor package used in electronic apparatus such as a portable computer tends to increase the number of terminals in such a semiconductor package. Recently, a BGA (Ball Grid Array) package is noticed to solve the problem and realize a high packing density.
A BGA package comprises a package substrate with previously mounted an IC chip and a plurality of solder balls arranged like a grid on the backside of the package substrate. These solder balls are soldered to a printed wiring board. A printed wiring board has a packaging area to mount a BGA package and a plurality of pads arranged in a grid. The pads have a plane circular surface and are soldered to the solder balls. In a conventional printed wiring board, all pads are equal in size (diameter) and are arranged at the same intervals. The pads are electrically connected to conductive patterns formed on the packaging area.
All pads are the same size in a conventional printed wiring board. Thus, as the intervals between the pads are reduced, only one conductive pattern can be formed in the narrow space between adjacent pads.
In this arrangement, when pulling the conductive patterns of all pads out of the packaging area, the conductive patterns connected to the pads of the second row from the outermost can be pulled out through the space among the first-row pads. However, the conductive patterns connected to the pads of the third and later rows cannot be pulled out through the space between the pads arranged after the third row.
Therefore, a multilayer printed wiring board with alternately laminated conductor and insulating layers is conventionally used. A multilayer printed wiring board has a plurality of through holes adjacent to the pads of the third and later rows at the middle of the packaging area. These through holes provide electrical connection between a plurality of inside conductor layers and the conductive patterns connected to the pads of the third and later rows.
With the above structure, a plurality of pads arranged at the middle of the packaging area is electrically connected to the inside conductor layers through a plurality of through holes. Thus, as the number of pads is increased, the number of conductor layers is also increased. This will increase the cost of the board. Moreover, the existence of through holes means that a conductor layer cannot be formed in some parts of the board. Thus, designing the conductor layer pattern and layout requires much time and effort.
Jpn. Pat. Appln. KOKAI Publication No. 2001-230533 disclosed a printed wiring board in which the shape of a pad to solder a solder ball is not a circle, but, elliptical having long and short axes, for example. In this printed wiring board, a large space can be taken between adjacent pads by modifying the arrangement of pads. Thus, a plurality of conductive patterns can be passed through adjacent pads, decreasing the number of layers and through holes.
However, in the above printed wiring board, the shape and direction of pads become different at a few locations on the packaging area. This requires much time and manpower in designing the pad and layout, provide little cost reduction compared to a conventional multilayer printed wiring board.
It is an object of the present invention to provide a printed wiring board which permits passing a plurality of conductive patterns between adjacent pads without complicating the pad shape and layout.
To achieve the above objects, a printed wiring board according to the present invention comprises a substrate having a packaging area to mount a circuit component with a plurality of connection terminals, a plurality of pads arranged in a grid in the packaging area of the substrate, and a plurality of conductive patterns connected to the pads. The pads are electrically connected to the connection terminals of the circuit component, and at least a few of the pads located at the periphery of the packaging area are made smaller than those located inside.
With this structure, the space between adjacent pads increases toward the periphery of the packaging area, and a plurality of conductive patterns can be easily passed through adjacent pads. Thus, a plurality of conductive patterns connected to the pads located inside of those at the periphery can be pulled out of the packaging area through adjacent pads.
Further, the pads located at the periphery and the others are different only in size, and the shape and the direction against the packaging area are the same. This facilitates the design and layout of the pads, and reduces the cost of the printed wiring board.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.